Method for heat absorption using polyoxymethylene polymer compositions

ABSTRACT

The present invention provides methods for heat absorption and thermally protected enclosures for containing heat sensitive devices, which include a heat absorption composition including polyoxymethylene polymer.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods and compositions useful for heat absorption, particularly in crash survivable protective enclosures for flight recorders used in aircraft and other vehicles operating over land and water. More particularly, the present invention relates to heat absorption compositions for use in a crash survivable enclosure which is capable of withstanding high impact, shock and mechanical penetration forces that commonly occur in a crash and the high temperatures, flames, and heat that often develop after a crash. The enclosure is designed to resist damage to solid state memory devices contained therein, even if the enclosure is subjected to immersion in fresh water or salt water for a prolonged period.

SUMMARY OF THE INVENTION

As a first aspect, the present invention provides a method for dissipating heat away from a heat sensitive device exposed to a high temperature environment. The method comprises surrounding the heat sensitive device with a composition comprising a polyoxymethylene polymer and optionally also a binder. The polyoxymethylene polymer undergoes endothermic decomposition when exposed to the high temperature environment. The composition absorbs heat from the high temperature environment during the endothermic decomposition of the polyoxymethylene polymer and dissipates heat away from the heat sensitive device.

As a second aspect, the present invention provides an enclosure for thermally protecting one or more heat sensitive devices from a high temperature environment. The enclosure comprises a) an outer housing having interior surfaces defining an interior cavity for containing one or more heat sensitive devices; and b) a heat absorption composition comprising a polyoxymethylene polymer. The composition occupies at least a portion of the interior cavity and substantially surrounds the heat sensitive devices. The polymer exhibits endothermic decomposition when the enclosure is subjected to the high temperature environment, and the composition absorbs heat from the high temperature environment during the endothermic decomposition of the polymer.

These and other aspects of the present invention are described further in the detailed description of the preferred embodiments of the invention which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the time versus temperature curve for the polyoxymethylene polymer compositions of the present invention as well as conventional heat absorption compositions employed in thermally protected enclosures such as flight data recorders.

FIG. 2 is a cross-section of a thermally protected enclosure according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, all terms have their ordinary meaning in the art unless otherwise indicated. All percentages are percent by weight unless otherwise indicated.

The methods of the present invention involve the use of certain polymer compositions which are characterized in that they absorb heat from a high temperature environment and thereby dissipate heat away from heat sensitive devices which are exposed to the high temperature environment. Such compositions are particularly useful in thermally protected enclosures such as flight data recorders. The compositions of the present invention, are designed to possess greater heat absorption, and thus heat dissipating, characteristics than conventional heat absorption compositions which are currently employed in flight data recorders to absorb and dissipate heat away from heat sensitive devices. The heat absorption characteristics of the composition are useful for providing thermal insulation to heat sensitive devices within the enclosure in a fire or other high temperature environment. In accordance with the present invention, the composition absorbs heat from the high temperature environment during the endothermic decomposition of the polyoxymethylene polymer. The absorption of heat from the high temperature environment effectively dissipates the heat away from the heat sensitive devices by controlling the temperature rise within the interior compartment of the flight data recorder. As a result, the temperature to which the heat sensitive devices are exposed does not rise to a level at which the heat sensitive devices will be damaged even though the temperature to which the flight data recorder is exposed is significantly higher than the temperature at which damage to the heat sensitive device will occur.

The temperature at which the endothermic decomposition occurs is selected to be: (a) above the peak temperature encountered under normal operating conditions; and (b) at or below the peak temperature objective for the heat sensitive devices within the enclosure which are desirously protected from the high temperature environment.

The compositions employed in the methods and enclosures of the present invention include a polyoxymethylene polymer having the general formula:

 H—(—O—CH₂—)_(n)—OH

where n is a number from 8 to 100 inclusive.

Polymers having the forgoing general formula are commercially available in the art and may be known as “paraformaldehyde”. Another polymer having the foregoing structural formula is commercially available under the tradename DELRIN® from DuPont. Those skilled in the polymer art may readily identify other suitable commercially available polymers having the foregoing general formula by reference to any of several chemical reagent supplier catalogues.

Certain commercially available polyoxymethylene polymers may be stabilized to prevent unzipping. Accordingly, when such-polymers are employed in the methods and compositions of the present invention, it may be desirable to include, in the composition, an acid catalyst capable of initiating the degradation or unzipping of the stabilized polyoxymethylene polymer. The required decomposition rate, which is a function of the heat flow-rate into the interior of the enclosure, will determine whether a catalyst is needed. In addition, the heat-flow rate will also determine how the catalyst should be included in the composition. If only a little catalytic activity is required, the catalyst may be applied as a dry power to the surface of the heat absorbing composition. If greater catalytic activity is required, it may be beneficial to disperse the acid catalyst throughout the polymer composition. Suitable acid catalysts which may be employed to catalyze the decomposition of the polyoxymethylene polymer in the high temperature environment include, catalysts which generate acids upon exposure to heat. Specific examples of acid catalysts include benzoic acid, diphenyliodonium triflate and triphenylsulfonium triflate. Typically, the acid catalyst is provided in an amount of from about 0 to about 5 percent by weight of the composition.

If desired, the heat absorption compositions of the present invention may include a binder, although a binder is not necessary. The polymers employed in the present invention are typically in the form of a powder, flake or other particulate form, which can be molded, extruded, or machined into a complex shape. However, a binder may be admixed with the particulate polymer to assist in holding the polymeric particles together during the molding process. For example it is often desirable for compositions for use in thermally protected enclosures to be molded into the general configuration of the interior cavity of the enclosure or smaller geometrically shaped units such as blocks and cubes. The binder holds the aggregated powder in the desired configuration. As a further advantage, the binder also provides shock absorption properties to the composition.

The binder is typically a wax binder. Examples of suitable wax binders include but are not limited to polyethylene wax, polypropylene wax, oxidized polyethylene, copolymers of ethylene and comonomer, and copolymers of propylene and comonomer. Suitable comonomers include those monomers which are copolymerizable with ethylene or propylene, such as linear, branched, or cyclic alkene monomers including butene, hexene, pentene, octene, and higher homologs; acrylic acid; and vinyl acetate. These binders, including the copolymers, are commercially available. Preferred binders include oxidized polyethylene wax, paraffin wax, and poly(ethylene-vinyl acetate). Oxidized polyethylene wax is currently the most preferred binder for use in the compositions of the present invention.

Typically, the composition includes a) between about 90 percent and about 100 percent by weight inclusive of the polyoxymethylene polymer; and b) between about 10 percent and about 0 percent by weight of the binder. More preferably the composition comprises about 95 percent by weight of a polymer and about 5 percent by weight of a binder. One preferred composition includes about 99 percent by weight of the polymer and about 1 percent by weight of the binder. In another preferred embodiment, the composition does not include binder, and includes 100 percent by weight of the polyoxymethylene polymer.

In those embodiments wherein a binder is included in the composition, the composition may be prepared by manually combining the polymer and binder in a manner that provides a relatively uniform composition. The composition may be prepared by manually combining the two components in a manner that provides a relatively uniform composition. The composition may be prepared by mechanically mixing, stirring or otherwise combining the components so as to provide a blend of uniform consistency. For example, the composition may be prepared by blending the binder into the particulate polymer component. Once a composition of uniform consistency is achieved, the composition may be molded into any suitable shape by using relatively low pressure. The blocks which are often desirable for use in flight data recorders may be prepared using compression molding techniques. For example blocks may be prepared by grinding the wax binder below room temperature into a fine powder, mixing the powdered wax binder with the polymer at a temperature of from between about 50 and about 95 at a pressure between about 5625 psi and about 22,500 psi. Typically, the higher temperatures and pressures are preferred in the compression molding of the compositions of the present invention.

In a fire or other high temperature environment, the compositions of the present invention act as a heat shield initially, by exhibiting thermal inertia. When the composition reaches the reactivity temperature, i.e., the temperature at which decomposition of the polymer begins, the composition acts as a heat sink by utilizing the heat energy of the high temperature environment to initiate and maintain the endothermic decomposition reaction of the polymer. The reactivity temperature of the composition is about 140° C.

Once the compositions of the present invention reach the reactivity temperature, the endothermic decomposition reaction begins. At temperatures equal to or greater than the reactivity temperature, the polyoxymethylene polymer undergoes a polymer “unzipping” reaction as follows.

H—(—O—CH₂—)_(n)—OH_nH₂C═O+H₂O.

In other words, the polymer decomposes back to its original monomers, namely formaldehyde. The polymer unzipping proceeds by an endothermic reaction mechanism. The term “endothermic” is well defined in the art and refers to reactions which require the input of heat energy to maintain the reacting mass at a constant temperature. Thus, the heat absorption composition absorbs heat from the high temperature environment to initiate and drive the endothermic decomposition of the polyoxymethylene polymer. The use of the heat energy to initiate and drive this endothermic reaction has the effect of maintaining the maximum temperature environment within the enclosure at an acceptable level which does not cause significant heat damage to the heat sensitive devices within the enclosure.

The formaldehyde product which results from the endothermic decomposition reaction is flammable. The amount of formaldehyde gas released from the composition is often significant. Without venting, the pressure could rise excessively and the heat dissipation properties would also be lower. To avoid the potential pressure damage to the heat sensitive devices within the enclosure as the pressure from the formaldehyde gas increases, it is desirable to provide a vent in the enclosure to allow the escape of the generated formaldehyde gas.

The expected gas evolution rate can be predicted from the heat flow anticipated in the final design. The heat flow can be estimated from the wall type and thickness, the insulation type and thickness (or thermal conduction coefficient) and the approximate physical size of the unit. Given the heat flow, the temperature profile and gas evolution rates can be estimated from each material's known thermochemical behavior.

FIG. 1 is a graphical representation of the time versus temperature characteristics of the compositions of the present invention as compared to sodium bicarbonate, another new heat absorption composition, which is described in U.S. Application Ser. No. 08/963,879, now U.S. Pat. No. 5,932,839 issued Aug. 3, 1999, filed concurrently herewith. As can be seen from the graph, the polyoxymethylene polymer compositions of the present invention absorb more heat than the sodium bicarbonate compositions, which in turn are more efficient than conventional heat absorption compositions. The time required for the polyoxymethylene polymer composition to reach a given temperature in a high temperature environment is as long or longer than the time required for the bicarbonate composition to reach the same temperature. This result demonstrates that the polyoxymethylene polymer compositions are as effective or more effective than the sodium bicarbonate at absorbing heat from the high temperature environment. The compositions of the present invention have an added advantage over conventional silica compositions in that they will not loose water in an unconfined space. An additional advantage over hydrated silica composition is ease of formability and subsequent handling during the manufacturing process.

The improved heat absorption properties observed with the polyoxymethylene polymer compositions of the present invention as compared to conventional heat absorption compositions is a very important advantage in applications such as flight data recorders. In the event of a crash, the flight data recorder may be subjected to temperatures in the range of 1100° C. for up to an hour. Thus it is clearly advantageous to use a heat absorption composition which absorbs more heat energy and thereby provides greater protection to the heat sensitive devices contained therein.

In addition to the foregoing, the compositions of the present invention provide a number of advantages. The components of the composition are relatively easy to handle, relatively low in cost, formable, and more effective at heat absorption as compared to conventional heat absorption materials. Also, because the compositions of the present invention absorb heat more efficiently than conventional heat absorption agents, flight data recorders including the compositions of the present invention may be smaller than conventional flight data recorders without sacrificing heat absorption protection for the heat sensitive devices contained therein.

The compositions of the present invention are particularly useful in enclosures for thermally protecting one or more heat sensitive devices from a high temperature environment, such as aircraft flight data recorders which include heat sensitive devices for recording and storing data. Flight data recorders are known in the art and are configured to provide a record of various important aircraft performance parameters over a predetermined time interval. Flight data recorders record performance parameters continuously from the time the electrical system of the aircraft is activated. Because they have a limited memory they can only hold information relating to such parameters for the predetermined time interval. Hence, as new data is continuously collected, it is continuously written over old data. In the event that the flight data recorder is deactivated, which occurs if the aircraft crashes, the flight data recorder will retain in memory the performance parameters recorded over the predetermined time interval immediately prior to deactivation. In operation, the information stored within a memory unit is continually supplied by additional components of the flight data recorder system such as a data acquisition unit that receives input signals from various aircraft sensors and systems, and processes those signals to produce signals compatible with the recording or storage medium employed by the flight data recorder memory unit.

The compositions of the present invention may be employed in any conventional thermally protected flight data recorder, including but not limited to the flight data recorders described in U.S. Pat. Nos. 4,694,119 and 4,944,401 both to Gruenwegan, U.S. Pat. No. 4,600,449 to White et al., and U.S. Pat. No. 5,438,162 to Thompson et al., the disclosures of which are hereby incorporated by reference in their entirety. Generally, the thermally protected flight data recorders are one type of thermally protected enclosures which include an outer housing, the heat sensitive devices for recording data, and the heat absorption (i.e., thermal protecting) composition of the present invention.

Referring to FIG. 2, the outer housing 15 of the enclosure 40 is typically constructed of a metal that exhibits a high thermal conductivity and high resistance to crushing and piercing. An intumescent coating or paint is generally applied to the exterior surfaces of the outer housing 15 for additional thermal insulation. The enclosure 10 may be provided in any suitable shape, and is typically substantially rectangular in cross-section when viewed perpendicular to each of its major axes. An insulating layer of solid material that exhibits a relatively low thermal conductivity adjoins each interior surface of the outer housing 15 to form a rectangular interior cavity that is centrally located within the outer housing 15. As noted above, the housing 15 may be designed to permit the escape of gaseous carbon dioxide which is produced in the endothermic decomposition of the bicarbonate compound.

The one or more heat sensitive devices 25 employed in the enclosure 10 are located within and contained by the interior cavity of the housing 15. In a preferred embodiment of the invention, the heat sensitive devices 25 are solid state electronic memory devices for storing data which is to be recovered from each of the solid state electronic memory devices following exposure of the enclosure 10 to a high temperature environment. Examples of such devices include semiconductor electronically erasable programmable read-only memory circuits. A data acquisition unit periodically supplies digital signals which are sequentially written into the semiconductor memory circuits so that the memory circuits store a sequence of digital words that is a time sampled data representation of the history for each parameter being monitored. Typically, with respect to currently employed techniques, data compression is generally employed to allow storing digital signals representative of a 15-30 minute time history for each monitored parameter.

To provide the heat shielding necessary to protect these heat sensitive devices 25 from a high temperature environment to which the enclosure 10 is subjected, the composition 20 of the present invention is provided in the open spaces of the interior cavity of the housing 15, substantially surrounding the heat sensitive devices 25. The composition 20 occupies at least a portion of the interior cavity. The compositions 20 of the present invention are formable under low pressure and at temperatures below the melting point of either component, as discussed above. Accordingly, the composition 20 may be molded to both conform to the shape of the interior cavity and to surround the heat sensitive devices therein.

The compositions of the present invention do not combust within the enclosure because the small amount of oxygen within the enclosure will be rapidly depleted or expelled. The compositions of the present invention also absorb radiant heat from any residual insulation and from the interior surfaces of the enclosure. This heat is subsequently carried outside the container and disposed of in the flame front rather than building up inside the small cavity containing the heat sensitive devices.

In addition to their utility as heat absorbing materials, the compositions of the present invention, particularly in those embodiments including a binder, are also useful in enclosures such as flight data recorders as shock absorbing materials. For example, the enclosure may be subjected to intense shock in the area of 3400 Gs on impact during crash conditions. Under such physical impact force, the compositions of the present invention absorb a portion of the shock by deforming enough to divert the impact shock wave away from the memory device, but not enough to penetrate any surrounding insulation creating voids that might become heat flux paths to the heat sensitive devices.

Although the essential elements of the enclosure include only the housing, the heat sensitive devices, and the composition of the present invention, one skilled in the art can envision a number of modifications to the enclosure which are based upon conventional technology in the art of flight data recorders, and are therefore contemplated by the instant invention. For example, it may be desirable to include as another element of the enclosure, a thermal insulating layer between the compositions of the present invention and the interior surfaces of the interior cavity. Suitable thermal insulating liners will be readily determinable by those skilled in the art and include a shell-like thermal liner positioned within the interior cavity adjacent to the interior surfaces of the interior cavity, which conforms to the geometric shape of the interior cavity. Preferably, the thermal liner is a unitary structure formed of a solid material that is a good thermal insulator (i.e., has a low thermal conductivity, K) and relatively low density. Examples of such materials include but are not limited to thermal insulators sold under the tradenames MIN-K 2000®, from Johns-Manville Co., of Denver CO, and MICROTHERM®, from Upton-Wirral Merseyside, England.

The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein. 

That which is claimed is:
 1. An apparatus comprising: an outer housing having interior surfaces defining an interior cavity; one or more heat sensitive devices contained within said interior cavity; a composition comprising a polyoxymethylene polymer substantially surrounding said one or more heat sensitive devices, exhibiting polymer endothermic decomposition when the apparatus is subjected to a high temperature environment and absorbing heat from said high temperature environment during said endothermic decomposition of said polymer; and a means to vent from said interior cavity to the exterior of the apparatus, gas released during said polymer endothermic decomposition.
 2. The apparatus according to claim 1, wherein said one or more heat sensitive devices is a solid state electronic memory device for storing data which is to be recovered from each of said solid state electronic memory devices following exposure of said enclosure to said high temperature environment.
 3. The apparatus according to claim 1, wherein said composition is formed in blocks which occupy at least a portion of said interior cavity.
 4. The apparatus according to claim 1, further comprising a thermal insulating liner between said composition and said interior surfaces of said interior cavity. 